The present invention relates to electron guns such as those used in color picture tubes, and particularly to an improved electron gun capable of providing dynamic beam shape modulation.
There are several different types of electron guns used in color picture tubes. One type of gun presently in wide use is an inline electron gun. An inline electron gun is designed to generate at least two, and preferably three, electron beams in a common plane and to direct the beams along convergent paths to a small area spot on the screen.
There has been a general trend toward inline color picture tubes with greater deflection angles so as to provide shorter tubes. However, in tubes with 110.degree. deflection, it has been found that the electron beams become excessively distorted as they are scanned toward the outer portions of the screen. Such distortions, commonly referred to as flare, appear on the screen of the tube as an undesirable low intensity tail or smear extending from a desirable intense core or spot. Such flare distortions are due, at least in part, to the effects of the fringe portions of the deflection field of the tube deflection yoke on a beam as it passes through the electron gun, and to the nonuniformities in the yoke deflection field itself.
When the yoke's fringe field extends into the region of the electron gun, as is usually the case, the beams may be deflected slightly off axis and into a more aberrated portion of an electron lens of the gun. The result is frequently a flare distortion of the electron beam spot which extends from the spot toward the center of the screen. This condition is particularly troublesome in tubes having self-converging yokes with a toroidal deflection coil, because of the relatively strong fringing of toroidal type coils.
Self-converging yokes are designed to have a nonuniform field in order to increasingly diverge the beams as the horizontal deflection angle increases. This nonuniformity also causes vertical convergence of the electrons within each individual beam. Thus, the beam spots are vertically overconverged at points horizontally displaced from the center of the screen, causing a vertically extending flare both above and below the beam spot.
The vertical flare due to both the effects of the yoke's fringe field in the region of the gun and to the nonuniform character of the yoke field itself is an undesirable condition which contributes to poor resolution of a displayed image on the screen.
It is known to provide nonsymmetrical electron gun electrodes to provide a desired astigmatism in the electron optics of the gun, to compensate for the above-described flare astigmatism. See, for example, U.S. Pat. No. 4,234,814, issued to Chen and Hughes on Nov. 18, 1980, which describes a screen grid electrode having an aperture of rectangular cross-section facing backward toward the cathode and of circular cross-section facing forward toward the screen. This astigmatic screen grid is of one piece, electrically speaking, and is energized during tube operation with a fixed DC bias voltage. While this gun does reduce flare astigmatism to a degree sufficient for some tubes, still further correction is desirable for other tubes, particularly very wide angle deflection tubes, especially where they are to be used to display printed matter near the corners of the screen.
It also is known to provide a two-part screen grid electrode comprising a first apertured plate having an elongated aperture and a second plate having a circular aperture. Such structure is disclosed in U.S. patent application Ser. No. 164,685, now U.S. Pat. No. 4,319,163, filed on June 30, 1980 for "ELECTRON GUN WITH DEFLECTION-SYNCHRONIZED ASTIGMATIC SCREEN GRID MEANS" by Chen. In operating this gun, the second plate is energized with a DC bias voltage and the first plate is energized with a DC bias voltage superposed with a dynamic signal synchronized with the horizontal and/or vertical deflection signals. One drawback of this design is that, unless the modulation voltage is very small, the brightness of the tube also is modulated by the dynamic voltage applied.
It is therefore desirable to develop an electron gun design that utilizes a dynamic voltage to reduce the problem of flare but does not permit brightness modulation as a result of the application of the dynamic voltage.